1. Field of the Invention
The present invention relates to an improvement on the so-called jumping developing device in which a thin layer of one-component magnetic developer formed on a developer bearing member is brought to the proximity of a surface bearing an electrostatic latent image to be developed whereby the developer is caused to jump to said surface from said thin developer layer by means of the electric force of said latent image to render the same visible.
2. Description of the Prior Art
There are already known various developing methods utilizing one-component developer. Among these particularly noteworthy is so-called toner transition development, in which one-component developer applied as a thin uniform layer on a developer bearing means is brought into facing relationship to an electrostatic latent image bearing surface with a small clearance therebetween whereby the developer is caused to jump from said developer bearing means to the image bearing surface by the electrostatic attractive force to perform image development, as disclosed in Japanese Patent Publication Sho41-9475 and in U.S. Pat. No. 2,839,400. Such developing method assures satisfactory development completely free from background fog as the developer is not attracted nor brought into contact with the non-image area having no image potential. Besides the absence of carrier particles eliminates the troubles arising from the change in mixing ratio thereof in the developer or from the time-dependent deterioration of the carrier particles.
Furthermore, the present applicant proposed novel developing methods as disclosed in the U.S. Patent Applications Ser. Nos. 938,101 now abandoned and 938,494 now abandoned and Ser. Nos. 58,434 now abandoned and 58,435 now U.S. Pat. No. 4,292,387, issued Sept. 29, 1981.
The former method is featured in positioning one-component magnetic developer, a developer bearing non-magnetic means and a magnetic field generating means in this order to form a thin uniform layer of said developer on said developer bearing means under the effect of the magnetic field generated by said magnetic-field generating means, and maintaining said thin developer layer in facing relation to an electrostatic latent image bearing surface with a small clearance therebetween thereby extending the developer corresponding to the image area by the electrostatic attractive force of said latent image to perform the image development. This method also is capable of providing a completely fog-free visible image as the development is carried out without contact of the developer with the non-image area.
The latter method is featured in positioning one-component magnetic developer, a developer bearing non-magnetic means and a magnetic field generating means in this order to form a thin uniform layer of said developer on said developer bearing means under the effect of the magnetic field generated by said magnetic-field generating means, and maintaining said thin developer layer in facing relation to an electrostatic latent image bearing surface with a small clearance therebetween, wherein the image development is effected by applying an AC developing bias voltage across said clearance between the image bearing surface and developer bearing means and changing said clearance in time. In this method the developer reaches also the non-image area in the initial stage to effect the development of halftone image but reaches only the image area in the later stage, thereby providing the image without background fog and with improved halftone reproducibility in comparison with the above-explained method.
As explained in the foregoing, the developing methods in which a thin layer of one-component developer is faced to the latent image bearing surface are far superior to other conventionally known methods in terms of the developing performance, image reproducibility, service life of the developer, etc.
Conventionally such thin developer layer has ordinarily been formed by employing a non-magnetic sleeve as the developer bearing member and by applying a thin layer of one-component magnetic developer thereon by means of a blade of a magnetic material and a strong magnetic field generated by magnetic poles positioned inside said sleeve. As an improvement to the above-mentioned method, the present applicant already proposed a developing device in which a magnetic brush of magnetic developer is formed on a magnetic brush roller or a developer-feeding first bearing member and is brought into contact with a developing roller or an image-developing second bearing member to form a thin developer layer on said developing roller.
FIG. 1 schematically shows such developing device, comprising a magnetic brush sleeve roller 1 constituting the first developer bearing member (hereinafter referred to as magnetic brush sleeve), a developing sleeve roller 2 constituting the second developer bearing member (hereinafter referred to as developing sleeve), a fixed magnet 3 positioned inside said magnetic brush sleeve 1, a magnetic brush defining blade 4, one-component insulating magnetic developer 5, a thin developer layer 6 applied on said developing sleeve 2, a fixed magnet 7 positioned inside said developing sleeve 2, and a developer hopper 8. Along with the rotation of said magnetic brush sleeve 1, a toner layer 5' supported thereon collides with the surface of the developing sleeve 2 to provide the insulating developer with frictional charge, and a thin developer layer 6 is formed on said developing sleeve 2 by means of mirror force and the like. The magnetic pole 7 positioned inside the developing sleeve is provided to achieve an effect of increasing the probability of contact between the developer in the magnetic brush 5' on the magnetic brush sleeve 1 and the surface of the developing sleeve 2 thereby increasing the applied density of the developer, and an effect of causing uniform sliding contact of the magnetic brush with the developing sleeve 2, as detailedly described in the foregoing application of the present applicant.
The thin developer layer 6 thus formed on the developing sleeve 2 is brought by the rotation thereof to the proximity of a photosensitive drum constituting the surface to be developed, thereby effecting so-called jumping development. Ordinary jumping development is featured by a relatively narrow clearance .alpha., in the order of 300.mu., between the photosensitive drum 9 and the developing roller 2. Although the conventional developing device for forming a thin developer layer on the developing sleeve by means of a magnetic blade and a magnetic field has necessitated a very narrow clearance, in the order of 200 to 500.mu., between the magnetic blade and the developing sleeve in consideration of the above-mentioned clearance of ca. 300.mu. between the photosensitive drum 9 and the developing sleeve 2, the developing device shown in FIG. 1 allows to significantly expand the minimum clearance .beta. between the developing sleeve 2 and the magnetic brush sleeve 1 approximately to a range of 500 to 2000.mu., thereby reducing the load applied to the developer in the developer applying area and thus resolving the developer aggregation or other troubles inevitable in the conventional developing device.
In FIG. 1 there is also shown an AC bias voltage source 10 disclosed in the aforementioned U.S. Patent Applications Ser. Nos. 58,434 and 58,435.
Although the device shown in FIG. 1 is thus improved in comparison with the conventional devices, it still contains certain drawbacks to be resolved.
Firstly, the developer layer formation on the developing sleeve 2 substantially depending upon the triboelectric charging of the developer tends to result in a somewhat deficient density thereof, eventually leading to an unstable image quality.
Secondly, foreign matter eventually trapped between the magnetic brush defining blade 4 and the magnetic brush sleeve 1 generates a lacking portion in the developer layer 5', which correspondingly results in a low-density imperfection in the form of a white streak, and such imperfection directly results in, in the jumping development, a defect in the developed image.
Thirdly the absence of a doctoring blade or the like directly defining the thickness of developer layer 6 on the developing sleeve 2 tends to result in an unstable thickness, eventually leading to an uneven developer layer occasionally containing an excessively thick portion with respect to the clearance between the photosensitive drum 9 and the developing sleeve 2 or giving rise to a thickness in excess of said clearance thus causing damage to the photosensitive drum 9.
Fourthly, the developer layer on the developing sleeve 2 may not be fully recovered to a sufficient density within one turn after the developer is consumed in the preceding developing process, leading to the formation of a ghost image of the preceding cycle, thus smearing the image quality.